The mechanism by which neurotransmitters modulate the activity of cultured neurons is being investigated. Explant cultures of cerebellum or hypothalamus are being utilized as test systems on which to study the effects of applied putative neurotransmitters. Inotophoretic application of amino acids, such as gamma amino butyric acid, glutamate, glycine, or homocysteate, or monoamines such as norepinephrine, histamine or dopamine, while simultaneously placing appropriate blocking drugs in the bathing medium, will identify the types of receptors present on these neurons. Electrical stimulation of focal sites in cultures will be used to activate endogenous synapses in cerebellar cultures; the transmitters active at these sites will be identified through the use of appropriate pharmacological criteria.